Renewable energies collected from thermal solar energy have a significant technological and economic importance in the domestic and industrial sectors. We have thermoelectric solar plants with solar energy concentration by means of cylindrical-parabolic mirrors on solar collection tubes among the most important technologies.
These systems require a maximum absorption of solar energy and the lowest possible energy loss. With this purpose, these systems are configured in vacuum tubes or similar structures that reduce conduction and convection losses and have highly absorbable coatings that absorb solar energy and low emissivity characteristics to decrease energy loss due to thermal radiation in the far infrared spectrum, wherein selective absorbing coatings play an essential role in the performance of the system.
Due to the foregoing, having an appropriate characterization system for absorbing tubes that allows measuring their coefficients of transmission and reflection with spectral resolution (maximum energy absorption and minimum energy loss) and also ensuring the uniformity of these values at all points of the tube becomes relevant. To achieve this, the device must be capable of precisely measuring the extreme values of the coefficients of reflection and transmission (near zero or near the unit), with spectral precision (around 10 nm) in the wavelengths required by the different reference standards and in an automated manner along different points of the tube.
A device that takes a spectral measurement of this type is called a spectrophotometer.
U.S. Pat. No. 4,669,873, Jun. 2, 1987, describes a spectrophotometer that measures the light transmitted, reflected, or radiated on at least one object or layer. The light is taken through fiberglass cable formed by a fiber matrix from the measuring point to the spectrophotometer. The object of the patent is to allow a high number of measuring points, independent from each other, or of several objects based on one single measurement and one single system of analysis, maintaining the precision of the measurement. The system uses its spectroscopic light scattering device, so the light of each fiber is scattered in its wavelengths over different points with a CCD (charge-coupled device), for example, and for each fiber there is a different angle, and therefore, a different CCD. We therefore have an array of fibers during detection, each one with its own CCD, for the individual and simultaneous spectroscopy of the different sample points collected by the different fibers used.
U.S. Pat. No. 4,921,351 of May 1990 describes a spectrophotometer that measures the reflectance of an object using the light of a pulsed tube Xe lamp to emit several flashes within the measuring time and filter ambient light through a high-pass filter.
U.S. Pat. No. 5,815,254, September 1998, describes an apparatus with two measuring modes, one for transmittance and the other for reflectance. The apparatus consists of two optical channels with a detector in each optical channel that measures direct light and reference light. The advantage of using only two channels is that it allows us to have a compact device for the field. Optical channels are preferably formed by optical fibers, and may also be formed by lenses and mirrors. Light is only injected from a fiber to obtain the measurement of transmittance. For the measurement of reflectance, the object is illuminated from several fibers, all of which are positioned in a circumference and introduced at 45°, so the reflection of all the fibers is collected in one single fiber to take the signal to the detector.
U.S. Pat. No. 6,559,941 of May 2003 describes a UV-VIS spectrophotomer with a pulsed Xe light source and solid-state detectors. The dynamic range in the detectors is reduced by modifying the power emitted in the pulses of the light source and the width of the slit at the input of the monochromator. This patent refers to the spatial non-uniformity of the pulses generated in the lamp, which must be considered in the design of the detection to place solid-state detectors instead of photomultipliers.
Patent US2009/0213371A1, of August 2009, describes a spectrophotometer with two detectors. which different spectral ranges overlap to cover a greater spectral range than the spectral range of each one of them individually. It describes a main module containing the light generation and light detection electronics and optics, and a measuring or testing module, both of which are connected by means of two optical fibers. The main module may be connected by USB to a PC. The measuring module is custom manufactured for each different object shape, and are exchanged. It only projects the light on the sample and collects reflected light, which returns to the main module for its detection through the fiber. It uses fiber bundle. The diffraction network is placed in the detection part of the main module, so the detectors are placed in different positions to collect the spectrum corresponding to the exit angle of the diffraction network, that is to say, two detectors are used but the light bundle is not divided (to avoid losing power); instead, the spectrums are spatially separated with the exit angle of the diffraction network.
None of the aforementioned devices or any other similar one device fulfill the necessary requirements for the automated characterization of absorbing tubes for solar collectors, either due to range, sensitivity, and/or mechanical configuration.